Carbon substances such as carbon fiber, carbon nanotube, and graphene have been known as conductive materials. Metallic nanomaterials such as metallic nanoparticles and metallic nanowires have also been known as conductive materials. Electric devices such as liquid crystal display devices and photoelectric conversion devices (for example, organic EL devices, solar batteries, optical sensors, and the like) have been developed using these conductive materials.
The conductive materials obtained using the carbon substances can significantly reduce the used amount of rare metals or the like. In some cases, these conductive materials even enable no metal to be used at all. The conductive materials obtained using the carbon substances have high flexibility and high mechanical strength. In addition, the conductive materials are chemically stable. Therefore, attention has been paid to them as promising conductive materials.
Although carbon substances have relatively high conductivities, in conductive materials obtained using the carbon substances, the electrical resistance in conduction between molecules is high. In the case where a conductive material obtained using the carbon substance is used as a transparent electrode with a large area, the electrical resistance becomes higher in comparison to that in an indium tin oxide (ITO) film having the same light transmission. In the case where a conductive material obtained using the carbon substance is used as a wire or the like with a long total length, the electric resistance is higher compared with a metal conductive material such as copper (Cu). By combining metallic nanomaterials having high conductivities with the carbon substance, the conductivity can be improved.
An ITO film is generally used as the transparent electrode in electric devices such as liquid crystal displays, solar cells, and organic EL devices. Aluminum (Al) which has a small work function, a magnesium (Mg) alloy which has a smaller work function, or the like is used as the negative electrode of photoelectric conversion devices. Although its efficiency is low, a photoelectric conversion device is also known, which uses an ITO film as a negative electrode and a metal having a large work function as a positive electrode.
Indium (In), which is a rare metal, is used for ITO films. Carbon nanotubes and thin films of planar graphene have been explored as inexpensive, stable, and flexible transparent electrodes obtained without using Indium (In).